1. Field of the Invention
The present invention relates to a nonvolatile semiconductor storage device that is electrically writable, readable, and erasable, and its manufacturing method. In particular, the present invention relates to a structure of a floating gate in the nonvolatile semiconductor storage device.
2. Description of the Related Art
The market in nonvolatile memories in which data can be electrically rewritten and data can be stored even after the power is turned off is expanding. Features of a nonvolatile memory lie in that its structure is similar to that of a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and a region capable of accumulating charges for a long period of time is provided over a channel formation region. The charge storage region is formed over an insulating layer and is electrically isolated from the surroundings; therefore, it is also referred to as a floating gate. A floating gate is surrounded by an insulator and thus electrically isolated from the surroundings; therefore, the floating gate has a characteristic that it retains charges injected thereinto. A gate electrode called a control gate is provided over the floating gate with an insulating layer therebetween. The control gate is distinguished from the floating gate because a predetermined voltage is applied to the control gate in writing or reading of data.
A floating gate type nonvolatile memory having such a structure has a mechanism of storing data by electrically controlling injection of charges into the floating gate and release of charges from the floating gate. Specifically, the injection of charges into the floating gate and the release of charges from the floating gate are carried out by applying high voltage between the control gate and a semiconductor layer in which a channel formation region is formed. Here, Fowler-Nordheim (F-N) tunneling current (in the case of NAND type) or thermoelectrons (in the case of NOR type) is/are said to flow in the insulating layer over the channel formation region. Thus, the insulating layer is also called a tunnel insulating layer.
A floating gate type nonvolatile memory is required to have properties of retaining the charges stored in a floating gate for 10 years or more to insure reliability. Accordingly, a tunnel insulating layer is required to be formed with a thickness such that tunneling current can flow and to have high insulation property thereby preventing leakage of the charges.
Further, the floating gate formed over the tunnel insulating layer is formed of silicon which is the same semiconductor material as the semiconductor layer in which the channel formation region is formed. Specifically, a method of forming a floating gate from polycrystalline silicon is widely used. For example, a polysilicon film deposited to a thickness of 400 nm is known (Reference 1: Japanese Published Patent Application No. 2000-58685 (p. 7, FIG. 7)).